The present invention relates generally to circuit elements, and more specifically, to methods and apparatus for forming a circuit element including one or more conductive traces at least partially embedded in a thermoplastic substrate.
Conductive traces typically are applied to a substrate, such as a silica wafer, using photolithography techniques that require many steps, including application of resist, masking and etching. These steps often use chemicals that are environmentally unfriendly. Conductive traces printed on substrates using printing presses typically are unstable and detach from the substrate when exposed to further processing, e.g., plating baths and solder reflow. The conductive traces also are typically not capable of being electroplated as they lack sufficient conductivity and first must be electrolessly plated, which is expensive as it requires two plating steps and is environmentally unfriendly. There exists a need for circuit elements and methods of manufacture that permit printing of conductive traces using printing presses, where the conductive traces are stable, e.g., capable of being electrolytically plated. The present invention addresses these needs and provides additional benefits and improvements.
A novel approach to the manufacture of circuit elements has now been discovered. By practicing the disclosed invention, the skilled practitioner can construct novel circuit elements having embedded conductive traces. The advantages of the present invention include stable and well-adhered conductive traces that can be printed using commercial printing presses and can withstand further use and processing such as plating and solder reflow.
In one aspect, the invention features a circuit element that includes a thermoplastic substrate, and a conductive trace at least partially embedded in the thermoplastic substrate. The thermoplastic substrate can be formed from thermoplastic polymers which include ethylene vinyl acetate, ethylene ethyl acetate, polyethylene, polypropylene, polycarbonate, polyimide, polyethylene naphthalate, polyphenylene sulfide, polyester, synthetic paper, polystyrene, and copolymers and combinations thereof.
The circuit element can further include a second substrate, wherein the second substrate is disposed adjacent to the thermoplastic substrate and opposite the conductive trace. The thermoplastic substrate can be hot melt coated, co-extruded or laminated onto the second substrate. The second substrate can be a second thermoplastic substrate having a second softening temperature that is higher than the softening temperature of the thermoplastic substrate. For example, the thermoplastic substrate can be formed from ethylene vinyl acetate, ethylene ethyl acetate, polyethylene, polypropylene, polycarbonate, copolymers or combinations thereof, and the second substrate can be formed from polyimide, polyethylene naphthalate, polyphenylene sulfide, polyester, synthetic paper, polystyrene, or copolymers thereof. The second substrate also can be formed from metal, metal foils, paper, glass, silica, and combinations thereof.
The circuit element can further include a third substrate disposed adjacent to the second substrate opposite the thermoplastic substrate. The third substrate can be a thermoplastic substrate. Optionally, a conductive trace can be at least partially embedded in the third thermoplastic substrate.
The conductive trace can include conductive particles having a particle size distribution having at least two modes. The conductive particles can include both conductive powder and conductive flakes. The conductive flakes typically have a mean aspect ratio between about 2 and about 50. The conductive trace can include conjugated conductive particles. The circuit element can include electrolytic conductive plating disposed on the conductive trace and/or a protective coating disposed on a surface of the circuit element.
In another aspect, the invention features a method of forming a circuit element. The method includes the steps of providing a thermoplastic substrate having a softening temperature (TS), printing a conductive ink onto the thermoplastic substrate to form a trace, and embedding the trace into the thermoplastic substrate by heating the thermoplastic substrate to a temperature above about the TS about the trace.
The method can include drying the conductive ink at a temperature less than about the TS to form a trace prior to embedding it into the thermoplastic substrate. The method also can include using a thermoplastic substrate that also includes one or more additional substrates disposed adjacent to the thermoplastic substrate. The method can include pre-heating the thermoplastic substrate to a temperature above about the TS, and allowing the thermoplastic substrate to cool to below about the TS, prior to printing the conductive ink onto the thermoplastic substrate.
The step of embedding the conductive trace into the thermoplastic substrate can be achieved by localized heating. For example, the conductive trace can be embedded into the thermoplastic substrate by induction heating the conductive material in the conductive ink.
The method also can include flashing off at least a portion of a vehicle of the conductive trace and/or conjugating at least a portion of a conductive material in the conductive trace.
The method can include cross-linking the thermoplastic substrate after embedding the conductive trace in the thermoplastic substrate. The thermoplastic substrate can be cross-linked by electron beam radiation. The method also can include printing solder onto the thermoplastic substrate, adding electrical components to the thermoplastic substrate, and heating the solder to a reflow temperature. The method can include electrolytically plating the conductive trace to form electrolytic conductive plating on the conductive trace and/or coating a surface of the circuit element with a protective coating.